Conventional magnetic resonance imaging (MRI) systems while producing excellent image quality, are heavy and immobile.
For example, conventional MRI systems utilizing superconducting magnet systems which are now common-place, require a significant amount of helium gas to be vented to the outside during emergencies. The reliance on a superconducting magnet system, and the requirement that the helium gas be vented to the outside are some of the main reasons why traditional MRI systems are non-portable. In essence, such MRI systems need to be permanently installed in dedicated room equipped with a suitable helium vent.
More recently, magnet designers have been focussing on creating ever stronger and more homogenous magnets. By way of example, magnetic field strengths of 7 T are now commercially available. Surprisingly, the field uniformity is only several parts per million. This is prompted by the desire to obtain evermore signal that can then be traded for desirable imaging attributes, such as increased spatial resolution or decreased scan time.
While stronger magnetic fields do improve signal, the latter condition, high magnetic-field uniformity, has come under some strain. Sequence innovations like short T2 imaging techniques disclosed in U.S. Pat. No. 5,025,216, for example, allow an MRI system to live with greater field inhomogeneities than ever before. However, one consequence of increasing field inhomogeneities is increased demands for radio-frequency (RF) transmit power, as the bandwidth one asks of the transmit system is increased. One attempt to work around this issue is disclosed in U.S. Pat. No. 7,403,006, which teaches a carefully designed frequency sweep.
A further problem that has received some attention over the years is of how to arrange magnet segments to achieve a desired magnetic field profile. For example, the circular Halbach array discovered by Klaus Halbach and described for example in U.S. Pat. Nos. 4,538,130 and 5,148,138, consists of a series of magnet segments arranged in a ring around the desired volume as is shown in FIG. 1. As can be seen, each magnet segment has a magnetization direction, and the magnet segments are arranged in a ring so that their magnetization directions are all aligned with a plane defined by the ring. Furthermore, the magnet segments are arranged to orient their magnetization directions to progressively make two rotations through adjacent magnet segments in one direction of the ring. As shown in FIG. 1, angle α is chosen to maximize its contribution to the magnetic field in the center of the ring, which turns out to be α=2β, where α is the angle the magnet segment's center location makes to the magnetic field axis oriented at the center of the ring.
Multiple Halbach rings may be used as taught in for example U.S. Pat. No. 5,148,138 to extend the magnetic field profile.
Magnet assemblies using these principles have been built for the purposes of MRI as taught in, for example Clarissa Zimmerman Cooley, Jason P. Stockmann, Brandon D. Armstrong, Mathieu Sarracanie, Michael H. Lev, Matthew S. Rosen, and Lawrence L. Wald, “Two-Dimensional Imaging in a Lightweight Portable MRI Scanner without Gradient Coils”, (2014) Magnetic Resonance in Medicine 00:1-12, and U.S. Pat. App. Pub. No. 2014/0111202. However, at 45 kg, the magnet assembly is fairly heavy for portable applications.
Other prior art documents of general interest include:                Lustig et al., “Sparse MRI: The Application of Compressed Sensing for Rapid MR Imaging”, (2007) Magnetic Resonance in Medicine, 58:1182-1195 (Lustig);        Tony Tadic and B. Gino Fallone, “Design and Optimization of Superconducting MRI Magnet Systems With Magnetic Materials”, (2012) IEEE Transactions on Applied Superconductivity, 22(2):4400107-4400107 (Tardic);        Fletcher, R., and Reeves, C. M., “Function minimization by conjugate gradients”, (1964) The Computer Journal, 149-154 (Fletcher);        U.S. Pat. No. 5,319,339 (Leupold);        U.S. Pat. No. 5,320,103 (Rapoport);        U.S. Pat. No. 5,550,472 (Richard);        U.S. Pat. No. 5,621,324 (Ota);        U.S. Pat. No. 5,631,616 (Ohta);        U.S. Pat. No. 5,659,250 (Domigan);        U.S. Pat. No. 5,717,333 (Frese);        U.S. Pat. No. 5,825,187 (Ohashi);        U.S. Pat. No. 5,900,793 (Katznelson);        U.S. Pat. No. 6,163,154 (Anderson);        U.S. Pat. No. 6,191,584 (Trequaffrini);        U.S. Pat. No. 6,275,128 (Aoki);        U.S. Pat. No. 6,351,125 (Westphal);        U.S. Pat. No. 7,023,309 (Laskaris);        U.S. Pat. No. 7,245,128 (Ando);        U.S. Pat. No. 7,345,479 (Park);        U.S. Pat. No. 7,403,006 (Garwood);        U.S. Pat. No. 7,760,059 (Higuchi);        U.S. Pat. No. 8,148,988 (Blumich);        U.S. Pat. No. 8,860,539 (Sakellariou); and        U.S. Pat. App. Pub. No. 2013/0088225 (Weller).        
Accordingly, there is continued need for improvements in MRI systems and MRI methods.